The field of the invention is the field of Radio Frequency (RF) Transponders (RF Tags), wherein one or more base stations communicates information to one or more RF Tags which may contain logic and memory circuits for storing information about objects, people, items, or animals associated with the RF Tags. The RF Tags can be used for identification and location (RFID Tags) of objects and to send information to the base station by modulating the load on an RF Tag antenna. More specifically, the invention relates to a radio frequency tagging system that allows multiple base stations to coordinate their activity. This allows the base stations to efficiently cover a larger zone than one base station while still meeting regulatory requirements.
RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60""s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a xe2x80x9cbase stationxe2x80x9d at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current would lead to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna can be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a xe2x80x9cpassivexe2x80x9d tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics. An active tag may also change the loading on the tag antenna for the purpose of transmitting information to the base station, or it may act as a transmitter to broadcast the information from the tag antenna directly to the base station.
The xe2x80x9crebroadcastxe2x80x9d of the incoming RF energy at the carrier frequency is conventionally called xe2x80x9cback scatteringxe2x80x9d, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed xe2x80x9cbackxe2x80x9d to the transmitting antenna.
In the 70""s, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor.
Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.
U.S. Pat. No. 5,214,410, hereby incorporated by reference, teaches a method for a base station to communicate with a plurality of Tags.
Prior art tags typically use a number of discrete components connected together with an antenna. However, to substantially reduce the cost of the tags, a single chip connected to an antenna must be used.
In a typical configuration, an application controller (ie work station, computer, microcomputer etc.) issues a command to the base station. The base station executes the command and may report results back to the application controller.
In some applications the zone in which RF tags may reside is larger than the zone covered by a single base station. Two or more base stations are required for coverage. In order to provide continuous coverage, the zones of two or more of the base stations must overlap.
It is important that only one base station of the group covering a large zone be transmitting at any one time. Two base stations transmitting simultaneously could jam a tag in the overlapped area trying to receive a transmission. Further, a transmitting base station could jam another base station receiver trying to detect a low level signal from a tag. The transmissions of the two or more base stations must be coordinated. There are, however, problems associated with the coordination of base stations. One way to coordinate the base stations would be for the application controller to sequentially issue commands to each base station in the group covering the large zone. Problems with having the application controller coordinate the base stations include:
1. The application controller must service each base station for each command, leading to a decrease in system performance. Requiring the application controller to continuously control many base stations could overload the processing capacity of the application controller.
2. It is desirable for one application controller to service many zones. In many cases, the base stations should run autonomously until a significant event occurs, such as a tag entering the zone. In a typical case, the application controller and base stations will be on a local area network. Causing the application controller to continuously service many base stations adds unnecessary network traffic, and could overload the capacity of the network.
3. With a network application controller in the processing loop, the switching time from one base station turning off to another turning on is long and indeterminate. Tags overlapping the RF field of two or more base stations will see the field drop, and the tags will then reactivate when the power comes up again. This event can cause tags to reset. Indeed, passive tags powered by the RF field will lose energy in the tag energy store and tag electronics will not have enough energy to continue functioning. Either event causes a tag to lose the information that it carries which describes the state of the tag. In the case of multiple tag identification, the tag state prevents a tag from being identified more than once in the algorithm. If a tag in overlapping the RF fields from two base station loses state during the field switching, it will be identified more than once, degrading performance.
Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,521,601; 5,528,222; 5,538,803; 5,550,547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106; 5,682,143; 5,729,201; 5,729,697;5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; and 5,821,859. U.S. Patent applications assigned to the assignee of the present invention include: application Ser. No. 08/626,820, filed: Apr. 3, 1996, entitled xe2x80x9cMethod of Transporting RF Power to Energize Radio Frequency Transpondersxe2x80x9d, by Heinrich, Zai, et al. (now U.S. Pat. No. 5,850,181); application Ser. No. 08/694,606 filed Aug. 9, 1996 entitled RFID System with Write Broadcast Capability by Cesar et al. (now U.S. Pat. No. 5,942,987); application Ser. No. 08/681,741 filed Jul. 29, 1996 entitled RFID Transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich, Goldman et al. (now U.S. Pat. No. 5,874,902); and application Ser. No. 09/153,617 1 filed Sep. 25, 1998, entitled RFID Interrogator Signal Processing System for Reading Moving Transponder, by Zai et al. (now U.S. Pat. No. 6,122,329) The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference.
An object of this invention is an improved system of two or more RF base stations covering a single zone.
An object of this invention is to allow two or more base stations, once initiated, to coordinate their activities without intervention from an application controller. This greatly decreases the processing requirements of the application controller and traffic on the network connecting the application controller to the base stations.
An object of this invention is to allow two or more base stations to coordinate the switching of RF fields on and off in a fast, deterministic manner, so that RF tags overlapping the RF fields will not detect the field switching.
The current invention adds one or more circuits for receiving and/or for generating external trigger signals to the signal generator controller which controls a base station RF signal generator. The external trigger signal is sent directly from one base station to another, without intervention of the application controller which controls both base stations.